Kiewit: Phase two of Oroville Dam repairs expected to be more difficult

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Dive Brief:

General contractor Kiewit has said it expects repair work on the second phase of California's Oroville Dam to be more difficult than its first, according to The Mercury News.

The phase will involve repairs to the dam's main and emergency spillways. Concrete finishing, joint sealing, drain and cleanup work for the main spillway is expected to be complete by the year's end, while work on the emergency spillway — which includes construction of a pile wall in bedrock below the spillway — should be complete by January. Kiewit will then perform additional concrete work around the emergency spillway in February.

The repairs' second phase will see the top 730 feet of the structure completely rebuilt with reinforced structural concrete. California's Department of Water Resources (DWR) said the main spillway can now handle 100,000 cubic-feet per second of water flow, but is not expected to be in use this season.

Dive Insight:

The cement mixture Kiewit is using for its repairs contains fly ash, which allows for more density and cohesion in the final mix. This mixture, according to the Los Angeles Times, is sturdier than the cement that was available when the dam was built nearly 50 years ago. The cement, however, can shrink during its strengthening process, creating hairline cracks.

These cracks led to questions from the Federal Energy Regulatory Commission (FERC) last month. The agency concluded that the cracks were not likely to have a negative impact on the spillway. However, according to KQED News, one expert said these cracks could lead to water intrusion and corrosion of the reinforcing steel underneath. Others have told the Times that the hairline cracks on the new spillway were too small to permit water to seep inside.

In February, the spillway failed during heavy rains, forcing 188,000 residents to be evacuated to avoid the potentially dangerous flooding. The DWR awarded a $275.4 million contract to Kiewit in April. That price tag has since climbed to $500 million, after Kiewit fully assessed the extent of the damage.